MemStub64

Hunt Hypothesis

The MemStub64 rule detects potential in-memory execution of malicious payloads by identifying suspicious memory stubs commonly used in advanced persistent threat (APT) activities. SOC teams should proactively hunt for this behavior in Azure Sentinel to identify early-stage adversaries leveraging in-memory techniques to evade traditional detection mechanisms.

YARA Rule

rule MemStub64 {
    meta:
        author = "Jaume Martin"
    condition:
        hash.md5(0, filesize) == "6b5b46d3212fc3fc5b455d9efd8d3ffa"
}

Deployment Notes

This YARA rule can be deployed in the following contexts:

• Microsoft Defender for Endpoint — Custom indicators / advanced hunting
• Email Gateway — Attachment scanning
• File Share Monitoring — Periodic scanning of shared drives
• YARA CLI — Manual threat hunting on endpoints
• Source Rule

False Positive Guidance

• Scenario: Windows Update or System Update Readiness Tool execution
  Description: Legitimate system maintenance tasks may load memory-resident components that match the MemStub64 YARA rule.
  Filter/Exclusion: Check for ProcessName containing wuauclt.exe or SystemUpdateReadiness.exe.

• Scenario: Scheduled Task for Microsoft Endpoint Protection (formerly Microsoft Security Essentials)
  Description: Antivirus or endpoint protection tools may load memory-resident stubs during scheduled scans.
  Filter/Exclusion: Filter by ProcessName containing mpsvc.exe or mpengine.exe.

• Scenario: PowerShell script execution for system diagnostics or configuration management
  Description: Administrators may use PowerShell to load memory-resident modules for diagnostics or configuration tasks.
  Filter/Exclusion: Check for ProcessName containing powershell.exe and filter by CommandLine containing -Command or -File.

• Scenario: Microsoft Hyper-V or Virtualization-related processes
  Description: Hyper-V or virtualization tools may use memory stubs for hypervisor communication or management.
  Filter/Exclusion: Filter by ProcessName containing vmcompute.exe or hypervisor.dll (via parent process).

• Scenario: Microsoft SQL Server or other database maintenance jobs
  Description: Database maintenance tasks may load memory-resident components for performance or integrity checks.
  Filter/Exclusion: Check for ProcessName containing sqlservr.exe or sqlagent.exe and filter by CommandLine containing -m or -E.